DIROFILARIA IMMITIS: CHARACTERISTICS, LIFE CYCLE, TRANSMISSION, TREATMENT - BIOLOGY - 2025

Dirofilaria immitis is a worm that belongs to the phylum Nematoda. It is characterized by having a round shape. These parasites infest some mammals, the main one of which is the dog. The parasite is found in the circulatory system of these animals, specifically in the right ventricle and in the pulmonary arteries.

In the animals it infects, this parasite causes a disease known as heartworm disease, which mainly affects heart and lung function. The prevention of this disease consists mainly in the eradication of the vector, which is a mosquito of the Culicidae family. If left untreated, this infection is life threatening.

Dirofilaria immitis. Joelmills

characteristics

The dog is the main host for Dirofilaria immitis. Source: Pixabay.com

-Species. Dirofilaria immitis

Morphology

Dirofilaria immitis is a round worm whose body is cylindrical and elongated. They are thin and have a whitish color. They have a tough, protective layer called the cuticle. In this, certain longitudinal and transverse striae are observed.

These parasites are sexually dimorphic, that is, there are certain morphological differences between female and male individuals.

Female

The female specimens are larger, being able to measure almost 30 cm. Its body ends in a rounded shape. It does not present any type of striking structure at the caudal level, nor does it have extensions in this area.

Male

The males are smaller, as they measure up to 20 cm. At its caudal end they have structures called spicules, a right and a left, which it uses for copulation. Likewise, its body ends in a curved end, with a shape similar to a spiral. It also has structures in its caudal part, similar to fins.

Biological cycle

Dirofilaria immitis, like any other parasite, requires a host in order to develop. The host of this parasite is the dog. Likewise, it also needs a vector, in which to develop part of its life cycle and that can transmit it to its definitive host.

Vector

The vector of this parasite is a mosquito, belonging to the Culicidae family. This family encompasses several genera. However, those most associated with this parasite are Culex, Anopheles, and Aedes.

Specialists have discovered the ability to transmit this parasite in one species of Culex, seven species of the genus Aedes and two species of the genus Anopheles.

Mosquito of the genus Anopheles, vector of Dirofilaria immitis. Source: Dunpharlain

Guest

The main host of this parasite is the dog. In an individual infected with this parasite, the worms are lodged in the pulmonary arteries, as well as in the right ventricle. There they reproduce and release microfilarial larvae (L1) into the bloodstream.

Inside the vector

When a mosquito of any of the aforementioned genera bites an infected animal, together with its blood, it is also acquiring microfilariae.

Inside the mosquito, the larvae travel from the intestine to the Malpighi tubules, where they undergo transformation. The first thing that happens is that the larva takes on a shape similar to that of a sausage. Later it undergoes a new transformation and passes from larval stage L1 to L2.

After a few days (12 days) the L2 larva passes to the L3 larval stage, which is considered its infectious form. This L3 larva travels through the body of the mosquito towards its salivary glands and proboscis.

Inside the host

When the mosquito bites a healthy individual, usually a dog, the L3 larvae enter the body through the wound caused by the bite. Within the body of the definitive host, in a period of about 7 days, these L3 larvae undergo a molt and transform into L4 larvae. This occurs at the level of the subcutaneous tissue of the animal.

However, the L4 larvae can remain in this stage for a long period of time. Some specialists even speak of up to 120 days. However, on average, about 70 days after the L3 larva enters the host's body, it finally reaches the L5 stage.

The larvae that are in this stage are capable of penetrating various tissues, so at some point, they are capable of reaching the general or systemic circulation and through this they are carried to the pulmonary arteries, where they fixate and develop the parasite already in its adult state.

Dirofilaria immitis life cycle. Source: Cú Faoil (text), Anka FriedrichDirecoes_anatomicas.svg: RhcastilhosMosquito gender en.svg: LadyofHatsderivative work: Anka Friedrich

In the pulmonary arteries and the right ventricle of the heart, adult worms can survive for a long period of time, up to about 7 years. They reach sexual maturity about six months after entering the host's body, which is when they begin to produce microfilariae.

These microfilariae begin to circulate through the bloodstream, until another mosquito bites the infected animal and the cycle begins again.

Transmission

The disease transmitted by the Dirofilaria immitis parasite is heartworm. The transmission mechanism of this disease is through the bite of some species of mosquito belonging to the genera Anopheles, Culex or Aedes.

In dogs, which are the main hosts, the parasite is transmitted through the bite, after having bitten another infected dog. In the case of humans, the parasite is transmitted through the bite of a mosquito that has bitten an infected dog.

This means that in humans there is no transmission scheme in which the mosquito bites an infected human and then bites a healthy one. Only after having bitten an infected dog does the mosquito transmit the parasite. This is because most of the larvae that enter the human body die in a short time.

In general, infection in humans is caused by a single worm (male or female), so that its reproduction is not possible to generate microfilariae that circulate freely in the blood.

Symptoms of infection

In dogs

As is well known, Dirofilaria immitis mainly affects dogs, generating in them certain symptoms of a heart and lung nature, among which we can mention:

- Difficulty breathing (dyspnea), which can be mild or severe.

- Cough that does not remit with anything and is persistent over time.

- Fainting after having made some kind of physical effort.

- Inapetence.

- Appearance of the so-called vena cava syndrome.

- Cardiac problems such as: arrhythmias, heart murmurs, vascular edema, among others.

- Shortness of breath that can lead to death.

In the human being

In humans, Dirofilaria immitis mainly affects the lung, since thanks to the pulmonary arteries it reaches that organ.

Despite this, in general, it is rare for humans to manifest symptoms. The parasite in lung tissue tends to encapsulate and form nodules that are asymptomatic. They are usually discovered in a routine examination through a chest X-ray and mistaken for a tumor.

In the case of people who manifest symptoms, they are the following:

- Fever.

- Pain in the chest.

- Cough with bloody expectoration.

Treatment

In dogs

Before prescribing the appropriate treatment, the veterinarian must perform a very careful evaluation of the animal, determining its level of severity.

The first step in treatment is the elimination of the Wolbachia bacteria that is found inside the parasites. The drug used to eliminate it is doxycycline.

At the same time, macrocyclic lactone is supplied, which aims to eliminate possible larvae that may have been recently inoculated into the animal. This medicine only affects larvae younger than 2 months.

Once these larvae are removed, another drug called melarsomine dihydrochloride is applied, which is an adulticide. This means that it attacks larvae older than 2 months and adult parasites.

Likewise, the surgical alternative is also contemplated in those dogs that have a large number of parasites.

Evidence of parasites in the dog's heart. Source: Alan R Walker

In the human being

In the case of humans, the treatment that is carried out when pulmonary nodules that can be caused by this parasite are detected is surgical resection. In general, humans do not require a prescription for any anthelmintic medication taken.

References

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2. Barcat, J. and Said, H. (1999). Pulmonary dirofilariasis. Medicine. 59 (2)
3. Curtis, H., Barnes, S., Schneck, A. and Massarini, A. (2008). Biology. Editorial Médica Panamericana. 7th edition.
4. Ettinger, S. and Feldman, E. (1995). Textbook of veterinary internal medicine. 4 ^th WB Saunders Company
5. Hickman, CP, Roberts, LS, Larson, A., Ober, WC, & Garrison, C. (2001). Integrated principles of zoology (Vol. 15). McGraw-Hill.
6. Sánchez, M., Calvo, P. and Mutis, C. (2011). Dirofilaria immitis: a zoonosis present in the world. Journal of Veterinary Medicine. 22